Organelles 2A

Structure of mitochondria

Double membrane, inner membrane folds called cristae, intermembrane space, and matrix

Organelle

Specialised structure within a cell that performs a specific function necessary for cell survival. You should be able to recognise organelles in diagrams and understand both their structure and function.

Nucleus function

The nucleus stores the cell’s genetic information (DNA) and acts as the control centre of the cell, regulating gene expression and coordinating cellular activities such as growth, metabolism, and division.

Nucleus structure

The nucleus is surrounded by a double membrane called the nuclear envelope. The nuclear envelope contains nuclear pores that regulate transport of RNA and proteins between the nucleus and cytoplasm. Inside the nucleus the DNA is organised as chromatin (DNA associated with proteins).

Nucleolus definition

The nucleolus is a dense structure located inside the nucleus where ribosomal RNA (rRNA) is produced and ribosome subunits are assembled.

Why the nucleolus stains darker

The nucleolus stains more densely than surrounding nuclear regions because it contains high concentrations of ribosomal RNA (rRNA) and ribosomal proteins involved in ribosome assembly.

Function of the nucleolus

The nucleolus produces rRNA and assembles ribosomal subunits, which are then exported through nuclear pores into the cytoplasm where they participate in protein synthesis.

Endoplasmic reticulum (ER) definition

The endoplasmic reticulum is a network of interconnected membranes continuous with the nuclear envelope that is involved in protein and lipid synthesis and transport within the cell.

Rough endoplasmic reticulum structure

The rough ER is studded with ribosomes on its cytosolic surface, giving it a rough appearance when viewed under a microscope.

Functions of the rough ER

The rough ER synthesises proteins that are destined for secretion, insertion into membranes, or transport to other organelles. It also performs protein folding and modification before proteins are transported to the Golgi apparatus.

Transport from rough ER

Proteins synthesised on the rough ER are packaged into transport vesicles that bud off from the ER membrane and travel to the Golgi apparatus for further processing.

Smooth endoplasmic reticulum definition

The smooth ER lacks ribosomes and is primarily involved in lipid metabolism and other biosynthetic processes.

Functions of smooth ER

The smooth ER synthesises lipids such as phospholipids and steroids, detoxifies drugs and toxins, stores calcium ions (particularly in muscle cells), and functions as a biosynthetic factory for various molecules.

Ribosomes definition

Ribosomes are molecular complexes that function as the sites of protein synthesis within cells.

Structure of ribosomes

Ribosomes are composed of ribosomal RNA (rRNA) and proteins and consist of a large subunit and a small subunit that assemble during translation.

Location of ribosomes

Ribosomes can be found free in the cytosol or bound to the surface of the rough endoplasmic reticulum.

Function of ribosomes

Ribosomes translate messenger RNA (mRNA) sequences into polypeptide chains, which fold to form functional proteins.

Golgi apparatus definition

The Golgi apparatus is an organelle responsible for modifying, sorting, and packaging proteins and lipids for transport to different cellular destinations.

Golgi apparatus structure

The Golgi apparatus consists of stacks of flattened membrane-bound sacs called cisternae.

Cis face of Golgi apparatus

The cis face is the receiving side of the Golgi apparatus where vesicles arriving from the endoplasmic reticulum fuse with the Golgi membranes.

Trans face of Golgi apparatus

The trans face is the shipping side of the Golgi apparatus where modified proteins and lipids are packaged into vesicles and sent to their destinations.

Functions of the Golgi apparatus

The Golgi apparatus modifies proteins, sorts them according to their destination, and packages them into vesicles for delivery to organelles, the plasma membrane, or secretion outside the cell.

Endomembrane system definition

The endomembrane system is a network of organelles that work together to synthesise, modify, and transport proteins and lipids within the cell.

Protein transport pathway in the endomembrane system

Proteins are synthesised on the rough ER, packaged into transport vesicles, delivered to the Golgi apparatus for modification and sorting, then sent in vesicles to the plasma membrane or other destinations.

Movement of vesicles in the cell

Vesicles are transported through the cytoplasm by motor proteins that move along microtubules of the cytoskeleton.

Bulk transport in cells

Bulk transport mechanisms allow cells to move large molecules or particles into or out of the cell using membrane-bound vesicles.

Exocytosis definition

Exocytosis is the process by which vesicles fuse with the plasma membrane to release their contents outside the cell.

Steps of exocytosis

A vesicle moves toward the plasma membrane, the vesicle membrane fuses with the plasma membrane, and the vesicle releases its contents into the extracellular space.

Examples of exocytosis

Exocytosis is used for processes such as hormone secretion and neurotransmitter release.

Endocytosis definition

Endocytosis is the process by which the plasma membrane invaginates and forms a vesicle that brings substances into the cell.

Steps of endocytosis

The plasma membrane folds inward, the membrane pinches off to form a vesicle, and the vesicle containing extracellular material enters the cell.

Phagocytosis definition

Phagocytosis is a specialised form of endocytosis in which a cell engulfs large particles or microorganisms.

Example of phagocytosis

Immune cells such as macrophages perform phagocytosis to engulf and destroy bacteria and other pathogens.

Membrane cycling definition

The plasma membrane is continuously recycled through the processes of endocytosis and exocytosis, maintaining the size and composition of the membrane.

Membrane cycling mechanism

Endocytosis removes membrane from the cell surface while exocytosis adds membrane back, ensuring membrane balance.

Lysosomes definition

Lysosomes are membrane-bound organelles containing digestive enzymes that break down macromolecules and cellular debris.

Functions of lysosomes

Lysosomes digest macromolecules, degrade damaged organelles, and destroy pathogens that enter the cell.

Connection between lysosomes and endocytosis

Endocytosis forms vesicles containing external material that subsequently fuse with lysosomes where digestive enzymes break down the contents.

Mitochondria function

Mitochondria are responsible for generating ATP through the process of cellular respiration.

Mitochondria structure

Mitochondria contain an outer membrane, an inner membrane folded into structures called cristae, an intermembrane space between the two membranes, and an internal compartment called the matrix.

Cristae definition

Cristae are folds of the inner mitochondrial membrane that increase the surface area for reactions involved in ATP production.

Unique features of mitochondria

Mitochondria contain their own circular DNA, possess ribosomes, and can divide independently of the cell, supporting the endosymbiotic theory.

Peroxisomes definition

Peroxisomes are organelles involved in detoxification reactions and the metabolism of fatty acids.

Functions of peroxisomes

Peroxisomes break down fatty acids, detoxify harmful substances, and carry out reactions that produce and degrade hydrogen peroxide.

Peroxisomal enzymes

Peroxisomes contain enzymes such as oxidases and catalase that allow them to carry out oxidative reactions and detoxification processes.

Chloroplasts definition

Chloroplasts are organelles found in plant cells that carry out photosynthesis, converting light energy into chemical energy.

Structure of chloroplasts

Chloroplasts have a double membrane and contain an internal membrane system of thylakoids, which are stacked into structures called grana, surrounded by a fluid matrix called the stroma.

Function of chloroplasts

Chloroplasts perform photosynthesis by converting light energy into chemical energy stored in glucose.

Chloroplasts in human cells

Human cells do not contain chloroplasts because humans do not perform photosynthesis.